Epigenetic modifications play an important role in the regulation of many cellular processes including cell proliferation, differentiation, and cell survival. Global epigenetic modifications are common in cancer, and include global changes in DNA and/or histone methylation, dysregulation of non-coding RNAs and nucleosome remodeling leading to aberrant activation or inactivation of oncogenes, tumor suppressors and signaling pathways. However, unlike genetic mutations which arise in cancer, these epigenetic changes can be reversed through selective inhibition of the enzymes involved. Several methylases involved in histone or DNA methylation are known to be dysregulated in cancer. Thus, selective inhibitors of particular methylases will be useful in the treatment of proliferative diseases such as cancer.
EZH2 (human EZH2 gene: Cardoso, C, et al; European J of Human Genetics, Vol. 8, No. 3 Pages 174-180, 2000) is the catalytic subunit of the Polycomb Repressor Complex 2 (PRC2) which functions to silence target genes by tri-methylating lysine 27 of histone H3 (H3K27me3). Histone H3 is one of the five main histone proteins involved in the structure of chromatin in eukaryotic cells. Featuring a main globular domain and a long N-terminal tail, Histones are involved with the structure of the nucleosomes, a ‘beads on a string’ structure. Histone proteins are highly post-translationally modified however Histone H3 is the most extensively modified of the five histones. The term “Histone H3” alone is purposely ambiguous in that it does not distinguish between sequence variants or modification state. Histone H3 is an important protein in the emerging field of epigenetics, where its sequence variants and variable modification states are thought to play a role in the dynamic and long term regulation of genes.
Increased EZH2 expression has been observed in numerous solid tumors including those of the prostate, breast, skin, bladder, liver, pancreas, head and neck and correlates with cancer aggressiveness, metastasis and poor outcome (Varambally et al. Nature 419:624-629, 2002; Kleer et al. Proc Natl Acad Sci USA 100:11606-11611, 2003; Breuer et al. Neoplasia 6:736-743, 2004; Bachmann et al. Prostate 65:252-259, 2005; Weikert et al. Int. J. Mol. Med. 16:349-353, 2005; Sudo et al. British Journal of Cancer 92:1754-1758, 2005; Bachmann et al. Journal of Clinical Oncology 24:268-273, 2006). For instance, there is a greater risk of recurrence after prostatectomy in tumors expressing high levels of EZH2, increased metastasis, shorter disease-free survival and increased death in breast cancer patients with high EZH2 levels (Varambally et al. Nature 419:624-629, 2002; Kleer et al. Proc Natl Acad Sci USA 100:11606-11611, 2003). More recently, inactivating mutations in UTX (ubiquitously transcribed tetratricopeptide repeats X), a H3K27 demethylase which functions in opposition to EZH2, have been identified in multiple solid and hematological tumor types (including renal, glioblastoma, esophageal, breast, colon, non-small cell lung, small cell lung, bladder, multiple myeloma, and chronic myeloid leukemia tumors), and low UTX levels correlate with poor survival in breast cancer suggesting that loss of UTX function leads to increased H3K27me3 and repression of target genes (Wang et al. Genes & Development 24:327-332, 2010). Together, these data suggest that increased H3K27me3 levels contribute to cancer aggressiveness in many tumor types and that inhibition of EZH2 activity may provide therapeutic benefit.
Numerous studies have reported that direct knockdown of EZH2 via siRNA or shRNA or indirect loss of EZH2 via treatment with the SAH hydrolase inhibitor 3-deazaneplanocin A (DZNep) decreases cancer cell line proliferation and invasion in vitro and tumor growth in vivo (Gonzalez et al., 2008, GBM 2009). While the precise mechanism by which aberrant EZH2 activity leads to cancer progression is not known, many EZH2 target genes are tumor suppressors suggesting that loss of tumor suppressor function is a key mechanism. In addition, EZH2 overexpression in immortalized or primary epithelial cells promotes anchorage independent growth and invasion and requires EZH2 catalytic activity (Kleer et al. Proc Natl Acad Sci USA 100:11606-11611, 2003; Cao et al. Oncogene 27:7274-7284, 2008).
Thus, there is strong evidence to suggest that inhibition of EZH2 activity decreases cellular proliferation and invasion. Accordingly, compounds that inhibit EZH2 activity would be useful for the treatment of cancer.
Latent human immunodeficiency virus (HIV) proviruses are silenced as the result of deacetylation and methylation of histones located at the viral long terminal repeat (LTR). Chromatin immunoprecipitation experiments using latently infected Jurkat T-cell lines demonstrated that EZH2 was present at high levels at the LTR of silenced HIV proviruses and was rapidly displaced following proviral reactivation. Knockdown of EZH2 induced up to 40% of the latent HIV proviruses. Knockdown of EZH2 also sensitized latent proviruses to external stimuli, such as T-cell receptor stimulation, and slowed the reversion of reactivated proviruses to latency. Similarly, cell populations that responded poorly to external stimuli carried HIV proviruses that were enriched in H3K27me3 and relatively depleted in H3K9me3. These findings suggest that PRC2-mediated silencing is an important feature of HIV latency and that inhibitors of histone methylation may play a useful role in induction strategies designed to eradicate latent HIV pools (Friedman et al. J. Virol. 85: 9078-9089, 2011). Additional studies have shown that H3K27 demethylation at the proviral promoter sensitizes latent HIV to the effects of vorinostat in ex vivo cultures of resting CD4+ T cells (Tripathy et al. J. Virol. 89: 8392-8405, 2015).